Storing baseline information in EEPROM

ABSTRACT

Pre-stored no-touch or no-hover (no-event) sensor output values can initially be used when a sensor panel subsystem is first booted up to establish an initial baseline of sensor output values unaffected by fingers or other objects touching or hovering over the sensor panel during boot-up. This initial baseline can then be normalized so that each sensor generates the same output value for a given amount of touch or hover, providing a uniform response across the sensor panel and enabling subsequent touch or hover events to be more easily detected. After the initial normalization process is complete, the pre-stored baseline can be discarded in favor of a newly captured no-event baseline that may be more accurate than the pre-stored baseline due to temperature or other variations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/298,227, filed Nov. 16, 2011, which is a continuation of U.S. patentapplication Ser. No. 12/890,274, filed Sep. 24, 2010 (now U.S. Pat. No.8,077,160, issued Dec. 13, 2011), which is a divisional of U.S. patentapplication Ser. No. 11/650,037, filed Jan. 3, 2007 (now U.S. Pat. No.8,054,296, issued Nov. 8, 2011), the entire disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

This relates to panels used as input devices for computing systems, andmore particularly, to the pre-storing of baseline no-touch sensor outputvalues in nonvolatile memory for use during boot-up so that the baselineis not corrupted by fingers or other objects touching the sensor panelduring boot-up.

BACKGROUND OF THE INVENTION

Many types of input devices are presently available for performingoperations in a computing system, such as buttons or keys, mice,trackballs, touch panels, joysticks, touch screens and the like. Touchscreens, in particular, are becoming increasingly popular because oftheir ease and versatility of operation as well as their decliningprice. Touch screens can include a touch panel, which can be a clearpanel with a touch-sensitive surface. The touch panel can be positionedin front of a display screen so that the touch-sensitive surface coversthe viewable area of the display screen. Touch screens can allow a userto make selections and move a cursor by simply touching the displayscreen via a finger or stylus. In general, the touch screen canrecognize the touch and position of the touch on the display screen, andthe computing system can interpret the touch and thereafter perform anaction based on the touch event.

Touch panels can include an array of touch sensors capable of detectingtouch events (the touching of fingers or other objects upon atouch-sensitive surface). Future panels may be able to detect multipletouches (the touching of fingers or other objects upon a touch-sensitivesurface at distinct locations at about the same time) and near touches(fingers or other objects within the near-field detection capabilitiesof their touch sensors), and identify and track their locations.Examples of multi-touch panels are described in Applicant's co-pendingU.S. application Ser. No. 10/842,862 entitled “Multipoint Touchscreen,”filed on May 6, 2004 and published as U.S. Published Application No.2006/0097991 on May 11, 2006, the contents of which are incorporated byreference herein.

Proximity sensor panels are another type of input device that caninclude an array of proximity sensors capable of detecting hover events(the no-touch, close proximity hovering of fingers or other objectsabove a surface but outside the near-field detection capabilities oftouch sensors) as well as touch events. Proximity sensor panels may alsobe able to detect multiple instances of hovering referred to herein asmulti-hover events (the hovering of fingers or other objects above asurface at distinct locations at about the same time). Examples of aproximity sensor, a proximity sensor panel, a multi-hover panel and acomputing system using both an multi-touch panel and proximity sensorsare described in Applicant's co-pending U.S. application Ser. No.11/649,998 entitled “Proximity and Multi-Touch Sensor Detection andDemodulation,” filed on Jan. 3, 2007, the contents of which areincorporated by reference herein.

Proximity sensor panels can be employed either alone or in combinationwith multi-touch panels. In addition, it is noted that some touchsensors, particularly capacitive touch sensors, can detect some hoveringor proximity. Proximity sensors, as referred to herein, are understoodto be distinct from touch sensors, including touch sensors that havesome ability to detect proximity. Multi-touch sensor panels capable ofdetecting multi-touch events and multi-hover sensor panels capable ofdetecting multi-hover events may collectively be referred to herein asmulti-event sensor panels.

Both touch sensor panels and proximity sensor panels can be formed as anarray of rows and columns of sensors. To scan a sensor panel, a stimuluscan be applied to one row with all other rows held at DC voltage levels,although it is possible to drive a group of rows simultaneously andderive the results for each row. When a row is stimulated, a modulatedoutput signal can appear on the columns of the sensor panel. The columnscan be connected to analog channels (also referred to herein as eventdetection and demodulation circuits) that may be implemented usinganalog and/or digital circuits. For every row that is stimulated, eachanalog channel connected to a column generates an output valuerepresentative of an amount of change in the modulated output signal dueto a touch or hover event occurring at the sensor located at theintersection of the stimulated row and the connected column. Afteranalog channel output values are obtained for every column in the sensorpanel, a new row is stimulated (with all other rows once again held atDC voltage levels), and additional analog channel output values areobtained. When all rows have been stimulated and analog channel outputvalues have been obtained, the sensor panel is said to have been“scanned,” and a complete “image” of touch or hover can be obtained overthe entire sensor panel. This image of touch or hover can include ananalog channel output value for every pixel (row and column) in thepanel, each output value representative of the amount of touch or hoverthat was detected at that particular location.

Thus, for example, if a finger touched down directly in the center of atouch panel, the resultant image of touch would include analog channeloutput values for the pixels located near the center of the panelindicative of touch events occurring at those pixels. The pixels withthese output values might be generally grouped together in a oval orellipsoidal, fingerprint-shaped cluster. Furthermore, the pixels in thecenter of that oval can have output values indicative of a greater ofdegree of touch than those pixels at the outside edges of the oval. Asimilar image of hover can be captured for a finger hovering over thecenter of the panel.

As mentioned above, a display screen can be located beneath the sensorpanel or integrated with the sensor panel. A user interface (UI)algorithm can generate a virtual keypad or other virtual input interfacebeneath the sensor panel that can include virtual buttons, pull-downmenus and the like. By detecting touch or hover events at locationsdefined by the virtual buttons, the UI algorithm can determine that avirtual button has been “pushed.” The magnitude of the analog channeloutput values, indicating the “degree” of touch or hover, can be used bythe UI algorithm to determine whether there was a sufficient amount oftouch or hover to trigger the pushing of the virtual button.

Ideally, a particular amount of touch or hover should generate an analogchannel output value of the same magnitude, and thus trigger acorresponding virtual button at the same level or amount of touch orhover, regardless of where the touch or hover event occurred on a sensorpanel. However, the electrical characteristics of the sensors in asensor panel are likely to vary due to processing variations,manufacturing tolerances and assembly differences (which can be due tothe location of the sensors in relation to the edges and shape of thesensor panel). For example, variations in the etching pattern for theITO, variations in the dielectric constant of the glass substrate, thepresence of microbubbles in the laminated stackup of materials that formthe sensor panel, routing differences in traces on the panel and flexcircuits connecting to the panel, or differences in the dielectricconstant of the cover plastic, can affect the magnitude of the analogchannel output values from location to location within the sensor panel.This can lead to false triggering of virtual buttons or non-triggeringof virtual buttons, and a difficult user experience as the userdiscovers that certain areas of the sensor panel require more or lesstouching, or closer or farther hovering in order to trigger a virtualbutton.

To provide a more uniform response from the sensor panel given the sameamount of touch or hover, the sensor panel can be calibrated ornormalized during boot-up of the computing device that uses the sensorpanel. This calibration process can involve scanning the entire sensorpanel to determine raw baseline (no-touch or no-hover) output values foreach sensor in the panel, and then subtracting out differences in theoutput values so that all sensor output values are normalized toapproximately the same value. With a normalized baseline, subsequenttouch or hover events can be more easily detected as increases in outputvalues as compared to the normalized no-touch or no-hover output values,and the same amount of touch or hover at any location in the sensorpanel is more likely to consistently trigger a corresponding virtualbutton.

However, this normalization process presumes that there are no fingersor other objects touching or hovering above the surface of the sensorpanel when the raw baseline output values are first obtained. If a user,for example, powers up an electronic device while the user's finger wastouching or hovering above the sensor panel, the raw baseline outputvalues for the sensors at which a touch or hover event is detected willbe higher than if no finger was present. The higher raw baseline outputvalues for those sensors will be incorrectly interpreted as no-touch orno-hover output values, and the normalization process will generateerroneous normalized baseline values for those sensors.

SUMMARY OF THE INVENTION

Pre-stored no-touch or no-hover (no-event) sensor output values caninitially be used when a sensor panel subsystem is first booted up toestablish an initial baseline of sensor output values unaffected byfingers or other objects touching or hovering over the sensor panelduring boot-up. This initial baseline can then be normalized so thateach sensor generates the same output value for a given amount of touchor hover, providing a uniform response across the sensor panel andenabling subsequent touch or hover events to be more easily detected.After the initial normalization process is complete, the pre-storedbaseline can be discarded in favor of a newly captured no-event baselinethat may be more accurate than the pre-stored baseline due totemperature or other variations.

To implement this baseline normalization procedure, the raw no-eventbaseline values for the sensor panel can be captured prior to finalassembly, such as at the factory, for example, and stored innon-volatile memory such as an EEPROM. During boot-up, rather thanscanning the sensor panel for the raw baseline, the pre-stored rawbaseline, unaffected by any finger or object that may be present, can beused. Offsets can then be computed so that they can be applied tosubsequently received output values (after boot-up is complete) togenerate normalized output values. A scan of the sensor panel to obtainraw sensor output values can then be performed. A finger or other objectcan then be detected because the raw sensor output values will be higherthan the pre-stored raw baseline. Alternatively, it can be determinedthat there is no finger of other object present due to the appearance ofrelatively flat raw output values (within a predetermined range of eachother), or if the output values are below some predetermined thresholdor within some predetermined range of the pre-stored raw baseline. If afinger or other object is in fact detected, no new baseline will becaptured until the finger or other object has been removed, and thevalues appear flat and/or fall below the threshold or within thepredetermined range of the pre-stored raw baseline. In some embodimentsof the invention, the detection of a finger or other object can be usedto initiate diagnostics modes that run internal tests on the device, orput the device into a certain operational mode. When it has beendetermined that no finger or other object is present, a new scan of thesensor panel can be performed, a new baseline can be obtained and thepre-stored raw baseline can be discarded.

In some embodiments of the invention when a pre-stored raw baseline isnot employed as described above, a global baseline inversion detectionalgorithm may be performed instead. This algorithm is based on theempirical observation that if a large object such as a palm is placed ona sensor panel, the sensor output values underneath the large objectwill increase as expected, but just outside the object, where there isno touching or hovering, the sensor output values will unexpectedlydecrease. If a conventional “minimum increment baseline adaptation”process is used to dynamically maintain a revised baseline using thelowest sensor output values obtained, the raw baseline values for theoutside sensors will be artificially lowered, and an inaccurate revisedbaseline will be generated.

The global baseline inversion detection algorithm, which is executedduring boot-up, can eliminate the capture of an inaccurate baseline asdescribed above. A scan of the entire sensor panel is first performedand a baseline is captured. Next, the number of sensor output values inthe entire image with values above the current normalized baseline valueare summed (above_baseline_sum), and the number of sensor output valuesbelow the normalized baseline are summed (below_baseline_sum). Adetermination is made whether the magnitude of below_baseline_sum ismuch greater than (>>) the magnitude of above_baseline_sum (greater thana predetermined margin) for several frames. If this condition is notsatisfied, the sensor panel is re-scanned and the sumsabove_baseline_sum and below_baseline_sum are re-computed. The baselineis recaptured only when the magnitude of below_baseline_sumis >>magnitude of above_baseline_sum, and this condition persists forseveral consecutive frames.

When a palm or other large object is present, the captured sensor outputvalues will be high for the sensors underneath the large object and lowfor the outside sensors, and an erroneous baseline will be captured. Aslong as the palm or other large object is present, because the erroneousbaseline is being used, the values for above_baseline_sum andbelow_baseline_sum will both be small, the magnitude ofbelow_baseline_sum will not be much greater than the magnitude ofabove_baseline_sum, and no new baseline will be captured. As a result,the baseline values for the sensors outside the large object will not bepushed down to an artificially low level. When the palm or other largeobject is removed, all output values will return to the correctbaseline. In other words, the output values for the sensors that wereunderneath the large object will drop, and the output values for thesensors outside the large object should rise. However, because that thecaptured erroneous baseline is still being used, the magnitude ofbelow_baseline_sum will be much greater than the magnitude ofabove_baseline_sum. If this condition persists for a few scans, thealgorithm concludes that the palm or other large object has definitivelybeen removed, and a new baseline can be captured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an exemplary computing system operable witheither a multi-touch panel, a multi-hover panel, or a combination ofboth according to one embodiment of this invention.

FIG. 2 a illustrates an exemplary capacitive multi-touch panel accordingto one embodiment of this invention.

FIG. 2 b is a side view of an exemplary pixel in a steady-state(no-touch) condition according to one embodiment of this invention.

FIG. 2 c is a side view of an exemplary pixel in a dynamic (touch)condition according to one embodiment of this invention.

FIG. 3 illustrates an exemplary proximity sensor panel that can includean array of LED/photodiode pairs, each pair representing a portion of aproximity sensor, according to one embodiment of this invention.

FIG. 4 is an illustration of an exemplary proximity sensor according toone embodiment of this invention.

FIG. 5 illustrates exemplary analog channel (event detection anddemodulation circuit) according to one embodiment of this invention.

FIG. 6 a is a graph of an exemplary sensor panel cross-section for onerow of sensors versus sensor output value, showing the raw baselineobtained and the effect of a subsequent event according to oneembodiment of this invention.

FIG. 6 b is a graph of an exemplary sensor panel cross-section for onerow of sensors versus sensor output value, showing the normalizedbaseline obtained and the effect of a subsequent event according to oneembodiment of this invention.

FIG. 6 c is a graph of an exemplary sensor panel cross-section for onerow of sensors versus sensor output value, showing the raw baselineobtained in error when an object is present at the sensor panel.

FIG. 6 d is a graph of an exemplary sensor panel cross-section for onerow of sensors versus sensor output value, showing an erroneousnormalized baseline obtained and the effect of a subsequent event.

FIG. 7 is a flowchart illustrating baseline normalization according toone embodiment of this invention.

FIG. 8 a illustrates the phenomenon of lowered output values for sensorslocated just outside a large object.

FIG. 8 b illustrates a global baseline inversion detection algorithmaccording to one embodiment of this invention.

FIG. 8 c illustrates a modified global baseline inversion detectionalgorithm according to one embodiment of this invention.

FIG. 8 d illustrates an exemplary periodic baseline adjustment algorithmaccording to one embodiment of this invention.

FIG. 9 a illustrates an exemplary mobile telephone that can include amulti-touch panel and/or a multi-hover panel, and a panel processorconfigured for implementing baseline normalization or global baselineinversion detection according to one embodiment of this invention.

FIG. 9 b illustrates exemplary digital audio/video player that caninclude a multi-touch panel and/or a multi-hover panel and a panelprocessor configured for implementing baseline normalization or globalbaseline inversion detection according to one embodiment of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description of preferred embodiments, reference is madeto the accompanying drawings which form a part hereof, and in which itis shown by way of illustration specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be used and structural changes may be made withoutdeparting from the scope of the preferred embodiments of the presentinvention.

Pre-stored no-touch or no-hover (no-event) sensor output values caninitially be used when a sensor panel subsystem is first booted up toestablish an initial baseline of sensor output values unaffected byfingers or other objects touching or hovering over the sensor panelduring boot-up. This initial baseline can then be normalized so thateach sensor generates the same output value for a given amount of touchor hover, providing a uniform response across the sensor panel andenabling subsequent touch or hover events to be more easily detected.After the initial normalization process is complete, the pre-storedbaseline can be discarded in favor of a newly captured no-event baselinethat may be more accurate than the pre-stored baseline due totemperature or other variations.

Although some embodiments of the invention may be described herein interms of the normalization of capacitive multi-touch sensor panels,embodiments of the invention may be generally applicable to other typesof multi-touch sensors that can include resistive touch sensors, surfaceacoustic wave touch sensors, electromagnetic touch sensors, near fieldimaging touch sensors, optical touch sensors and the like. In addition,although the proximity sensors may be described herein as infrared (IR)proximity sensors, embodiments of the invention may be generallyapplicable to other types of proximity sensors having an output that canbe AC-coupled to an analog channel.

Multi-touch touch-sensitive panels may be able to detect multipletouches (touch events or contact points) that occur at about the sametime, and identify and track their locations. Similarly, multi-hoverhover-sensitive panels may be able to detect multiple occurrences ofhovering (hover events) that occur at about the same time, and identifyand track their locations. FIG. 1 illustrates exemplary computing system100 operable with either multi-touch panel 124, multi-hover panel 136,or a combination of both. Computing system 100 can include one or morepanel processors 102 and peripherals 104, and panel subsystem 106. Theone or more processors 102 can include, for example, an ARM968processors or other processors with similar functionality andcapabilities. However, in other embodiments, the panel processorfunctionality can be implemented instead by dedicated logic such as astate machine. Peripherals 104 can include, but are not limited to,random access memory (RAM) or other types of memory or storage, watchdogtimers and the like.

Panel subsystem 106 can include, but is not limited to, one or moreanalog channels 108, channel scan logic 110 and driver logic 114.Channel scan logic 110 can access RAM 112, autonomously read data fromthe analog channels and provide control for the analog channels. Thiscontrol can include multiplexing columns of multi-touch panel 124 ormulti-hover panel 136 to analog channels 108. In addition, channel scanlogic 110 can control the driver logic and stimulation signals beingselectively applied to rows of multi-touch panel 124 or multi-hoverpanel 136. Compensation hardware 138 in panel subsystem 106, UIalgorithm 140 in software or firmware executed by panel processor 102and/or channel scan logic 110 (which may be referred to collectivelyherein as simply UI logic) can be used to perform the sensor panelcompensation according to embodiments of the invention. In someembodiments, panel subsystem 106, panel processor 102 and peripherals104 can be integrated into a single application specific integratedcircuit (ASIC).

Driver logic 114 can provide multiple panel subsystem outputs 116 andcan present a proprietary interface that drives high voltage driver,which is comprised of decoder 120 and subsequent level shifter anddriver stage 118, although level-shifting functions could be performedbefore decoder functions. Level shifter and driver 118 can provide levelshifting from a low voltage level (e.g. CMOS levels) to a higher voltagelevel, providing a better signal-to-noise (S/N) ratio for noisereduction purposes. Decoder 120 can decode the drive interface signalsto one out of N outputs, whereas N is the maximum number of rows in thepanel. Decoder 120 can be used to reduce the number of drive linesneeded between the high voltage driver and panel 124. Each panel rowinput 122 can drive one or more rows in panel 124. In some embodiments,driver 118 and decoder 120 can be integrated into a single ASIC.However, in other embodiments driver 118 and decoder 120 can beintegrated into driver logic 114, and in still other embodiments driver118 and decoder 120 can be eliminated entirely.

Computing system 100 can also include host processor 128 for receivingoutputs from panel processor 102 and performing actions based on theoutputs that can include, but are not limited to, moving an object suchas a cursor or pointer, scrolling or panning, adjusting controlsettings, opening a file or document, viewing a menu, making aselection, executing instructions, operating a peripheral deviceconnected to the host device, answering a telephone call, placing atelephone call, terminating a telephone call, changing the volume oraudio settings, storing information related to telephone communicationssuch as addresses, frequently dialed numbers, received calls, missedcalls, logging onto a computer or a computer network, permittingauthorized individuals access to restricted areas of the computer orcomputer network, loading a user profile associated with a user'spreferred arrangement of the computer desktop, permitting access to webcontent, launching a particular program, encrypting or decoding amessage, and/or the like. Host processor 128 can also perform additionalfunctions that may not be related to panel processing, and can becoupled to program storage 132 and display device 130 such as a liquidcrystal display (LCD) for providing a UI to a user of the device.

Multi-touch panel 124 can in some embodiments include a capacitivesensing medium having a plurality of row traces or driving lines and aplurality of column traces or sensing lines, although other sensingmedia may also be used. The row and column traces can be formed from atransparent conductive medium such as Indium Tin Oxide (ITO) or AntimonyTin Oxide (ATO), although other transparent and non-transparentmaterials, such as copper, can also be used. In some embodiments, therow and column traces can be formed on opposite sides of a dielectricmaterial, and can be perpendicular to each other, although in otherembodiments other non-orthogonal or non-Cartesian orientations arepossible. For example, in a polar coordinate system, the sensing linescan be concentric circles and the driving lines can be radiallyextending lines (or vice versa). It should be understood, therefore,that the terms “row” and “column,” “first dimension” and “seconddimension,” or “first axis” and “second axis” as used herein areintended to encompass not only orthogonal grids, but the intersectingtraces of other geometric configurations having first and seconddimensions (e.g. the concentric and radial lines of a polar-coordinatearrangement). It should also be noted that in other embodiments, therows and columns can be formed on a single side of a substrate, or canbe formed on two separate substrates separated by a dielectric material.In some embodiments, the dielectric material can be transparent, such asglass, or can be formed from other materials such as Mylar. Anadditional dielectric cover layer may be placed over the row or columntraces to strengthen the structure and protect the entire assembly fromdamage.

At the “intersections” of the traces, where the traces pass above andbelow (i.e. cross) each other (but do not make direct electrical contactwith each other), the traces essentially form two electrodes (althoughmore than two traces could intersect as well). Each intersection of rowand column traces can represent a capacitive sensing node and can beviewed as picture element (pixel) 126, which can be particularly usefulwhen multi-touch panel 124 is viewed as capturing an “image” of touch.(In other words, after multi-touch subsystem 106 has determined whethera touch event has been detected at each touch sensor in the multi-touchpanel, the pattern of touch sensors in the multi-touch panel at which atouch event occurred can be viewed as an “image” of touch (e.g. apattern of fingers touching the panel).) The capacitance between row andcolumn electrodes appears as a stray capacitance on all columns when thegiven row is held at DC and as a mutual capacitance Csig when the givenrow is stimulated with an AC signal. The presence of a finger or otherobject near or on the multi-touch panel can be detected by measuringchanges to Csig. The columns of multi-touch panel 124 can drive one ormore analog channels 108 (also referred to herein as event detection anddemodulation circuits) in multi-touch subsystem 106. In someembodiments, each column is coupled to one dedicated analog channel 108.However, in other embodiments, the columns may be couplable via ananalog switch to a fewer number of analog channels 108.

FIG. 2 a illustrates exemplary capacitive multi-touch panel 200. FIG. 2a indicates the presence of a stray capacitance Cstray at each pixel 202located at the intersection of a row 204 and a column 206 trace(although Cstray for only one column is illustrated in FIG. 2 forpurposes of simplifying the figure). Note that although FIG. 2 aillustrates rows 204 and columns 206 as being substantiallyperpendicular, they need not be so aligned, as described above. In theexample of FIG. 2 a, AC stimulus Vstim 214 is being applied to one row,with all other rows connected to DC. The stimulus causes a charge to beinjected into the column electrodes through mutual capacitance at theintersecting points. This charge is Qsig=Csig×Vstm. Each of columns 206may be selectively connectable to one or more analog channels (seeanalog channels 108 in FIG. 1).

FIG. 2 b is a side view of exemplary pixel 202 in a steady-state(no-touch) condition. In FIG. 2 b, an electric field of electric fieldlines 208 of the mutual capacitance between column 206 and row 204traces or electrodes separated by dielectric 210 is shown.

FIG. 2 c is a side view of exemplary pixel 202 in a dynamic (touch)condition. In FIG. 2 c, finger 212 has been placed near pixel 202.Finger 212 is a low-impedance object at signal frequencies, and has anAC capacitance Cfinger from the column trace 204 to the body. The bodyhas a self-capacitance to ground Cbody of about 200 pF, where Cbody ismuch larger than Cfinger. If finger 212 blocks some electric field lines208 between the row and column electrodes (those fringing fields thatexit the dielectric and pass through the air above the row electrode),those electric field lines are shunted to ground through the capacitancepath inherent in the finger and the body, and as a result, the steadystate signal capacitance Csig is reduced by ΔCsig. In other words, thecombined body and finger capacitance act to reduce Csig by an amountΔCsig (which can also be referred to herein as Csig_sense), and can actas a shunt or dynamic return path to ground, blocking some of theelectric fields as resulting in a reduced net signal capacitance. Thesignal capacitance at the pixel becomes Csig−ΔCsig, where Csigrepresents the static (no touch) component and ΔCsig represents thedynamic (touch) component. Note that Csig−ΔCsig may always be nonzerodue to the inability of a finger, palm or other object to block allelectric fields, especially those electric fields that remain entirelywithin the dielectric material. In addition, it should be understoodthat as a finger is pushed harder or more completely onto themulti-touch panel, the finger can tend to flatten, blocking more andmore of the electric fields, and thus ΔCsig can be variable andrepresentative of how completely the finger is pushing down on the panel(i.e. a range from “no-touch” to “full-touch”).

Referring again to FIG. 2 a, as mentioned above, Vstim signal 214 can beapplied to a row in multi-touch panel 200 so that a change in signalcapacitance can be detected when a finger, palm or other object ispresent. Vstim signal 214 can include one or more pulse trains 216 at aparticular frequency, with each pulse train including a number ofpulses. Although pulse trains 216 are shown as square waves, otherwaveshapes such as sine waves can also be employed. A plurality of pulsetrains 216 at different frequencies can be transmitted for noisereduction purposes to detect and avoid noisy frequencies. Vstim signal214 essentially injects a charge into the row, and can be applied to onerow of multi-touch panel 200 at a time while all other rows are held ata DC level. However, in other embodiments, the multi-touch panel may bedivided into two or more sections, with Vstim signal 214 beingsimultaneously applied to one row in each section and all other rows inthat region section held at a DC voltage.

Each analog channel coupled to a column measures the mutual capacitanceformed between that column and the row. This mutual capacitance iscomprised of the signal capacitance Csig and any change Csig_sense inthat signal capacitance due to the presence of a finger, palm or otherbody part or object. These column values provided by the analog channelsmay be provided in parallel while a single row is being stimulated, ormay be provided in series. If all of the values representing the signalcapacitances for the columns have been obtained, another row inmulti-touch panel 200 can be stimulated with all others held at a DCvoltage, and the column signal capacitance measurements can be repeated.Eventually, if Vstim has been applied to all rows, and the signalcapacitance values for all columns in all rows have been captured (i.e.the entire multi-touch panel 200 has been “scanned”), a “snapshot” ofall pixel values can be obtained for the entire multi-touch panel 200.This snapshot data can be initially saved in the multi-touch subsystem,and later transferred out for interpretation by other devices in thecomputing system such as the host processor. As multiple snapshots areobtained, saved and interpreted by the computing system, it is possiblefor multiple touches to be detected, tracked, and used to perform otherfunctions.

FIG. 3 illustrates an exemplary proximity sensor panel 306 that caninclude an array of LED/photodiode pairs 300, each pair representing aportion of a proximity sensor, according to some embodiments of theinvention. In FIG. 3, each LED/photodiode pair 300 in a particular rowcan be simultaneously stimulated by Vstim 302 with the other rows heldat a DC voltage, and after a snapshot of the row has been captured,LED/photodiode pairs 300 in a new row can be stimulated. In the firsttwo columns of FIG. 3, each LED/photodiode pair 300 in a particularcolumn can be simultaneously connected to a single photodiode amplifier304, and each photodiode amplifier 304 can be connected to a separateanalog channel of the same design that can be used to detect changes insignal capacitance in a capacitive touch sensor array. In this manner,for every row being stimulated, the analog channels for each column candetermine, at about the same time, whether the LED/photodiode pair inthe row being stimulated has detected the presence of a finger, palm orother object. Eventually, if Vstim has been applied to all rows, and theeffect of any photodiode current on all columns in all rows has beencaptured (i.e. the entire proximity sensor panel 306 has been“scanned”), a “snapshot” of all pixel values can be obtained for theentire panel. This snapshot data can be initially saved in the panelsubsystem, and later transferred out for interpretation by other devicesin the computing system such as the host processor. As multiplesnapshots are obtained, saved and interpreted by the computing system,it is possible for multiple hover events to be detected, tracked, andused to perform other functions.

FIG. 4 is an illustration of exemplary proximity sensor 400 according tosome embodiments of this invention. Proximity sensors 400 can detect oneor more fingers, a palm or other object touching the multi-touch panelor hovering over the multi-touch panel in the far field without touchingit. Proximity sensor 400 can include source Vstim 402 that drives IRlight emitting diode (LED) 404, which emits transmitted IR 406. Vstim402 can include a burst of square waves in an otherwise DC signal, in amanner similar to the Vstim applied to the rows on the capacitivemulti-touch panel as described above, although in some embodiments thesquare waves representing Vstim can be preceded and followed by othernon-DC signaling. Reflected IR 408, which may have reflected off of afinger, palm or other object 410, can be detected by photodiode (e.g. afast pin diode) 412 or any other device (e.g. a phototransistor or othersensing device) whose current changes as a function of received IRlight. Photodiode 412 can be reversed biased to a reference voltageVref, which can be maintained at the − input (inverting input) ofphotodiode amplifier 414 whose + input (non-inverting input) is tied toVref. The photocurrent produced through the photodiode, Iphoto, alsoprimarily passes through the parallel combination of feedback resistorRfb and capacitor Cfb, and the output of the photodiode amplifier isVref−(Zcfb×Rfb)×(Iphoto+Iin)/(Zcfb+Rfb), the latter term(Zcfb×Rfb)×(Iphoto+Iin)/(Zcfb+Rfb), representing the voltage drop acrossRfb and Cfb where Iin is the input current to the inverting input ofphotodiode amplifier 414 and is usually negligible. The impedance Zcfbis frequency dependent and can be adjusted to optimize the gain of thephoto amplifier for a given modulation frequency of the signal Iphoto,whereas Iphoto(t)=Ip×sin(wt) with wt=2×PI×f mod and f mod is themodulation signal, Ip is the amplitude of the modulation signal andZcfb=−1/(jwt). The modulation frequency f mod is equivalent to themodulation frequency fstm of Vstm. The output of photodiode amplifier414 can be AC coupled using AC coupling capacitor 416.

FIG. 5 illustrates exemplary analog channel (event detection anddemodulation circuit) 500. One or more analog channels 500 can bepresent in the panel subsystem. One or more columns from a multi-touchpanel or a multi-hover panel can be connectable to input 540 of eachanalog channel 500. Each analog channel 500 can include virtual-groundcharge amplifier 502, signal mixer 504, offset compensation 506,rectifier 532, subtractor 534, and analog-to-digital converter (ADC)508. FIG. 5 also shows, in dashed lines, the steady-state signalcapacitance Csig that can be contributed by a multi-touch panel columnconnected to analog channel 500 when an input stimulus Vstim is appliedto a row in the multi-touch panel and no finger, palm or other object ispresent, and the dynamic signal capacitance Csig−ΔCsig that can appearwhen a finger, palm or other object is present.

Vstim, as applied to a row in the multi-touch panel, can be generated asa burst of square waves or other non-DC signaling in an otherwise DCsignal, although in some embodiments the square waves representing Vstimcan be preceded and followed by other non-DC signaling. If Vstim isapplied to a row and a signal capacitance is present at a columnconnected to analog channel 500, the output of charge amplifier 502 canbe pulse train 510 centered at Vref with a peak-to-peak (p-p) amplitudein the steady-state condition that is a fraction of the p-p amplitude ofVstim, the fraction corresponding to the gain of charge amplifier 502.For example, if Vstim includes 18V p-p pulses and the gain of the chargeamplifier is 0.1, then the output of the charge amplifier can be 1.8Vp-p pulses. This output can be mixed in signal mixer 304 withdemodulation waveform Fstim 516.

Because Vstim can create undesirable harmonics, especially if formedfrom square waves, demodulation waveform Fstim 516 can be a Gaussiansine wave in an otherwise DC signal that is digitally generated fromlook-up table (LUT) 512 or other digital logic and synchronized toVstim. In some embodiments, Fstim 516 can be tunable in frequency andamplitude by selecting different digital waveforms in LUT 512 orgenerating the waveforms differently using other digital logic. Signalmixer 504 can demodulate the output of charge amplifier 510 bysubtracting Fstim 516 from the output to provide better noise rejection.Signal mixer 504 can reject all frequencies outside the passband, whichcan in one example be about +/−30 kHz around Fstim. This noise rejectioncan be beneficial in noisy environment with many sources of noise, suchas 802.11, Bluetooth and the like, all having some characteristicfrequency that can interfere with the sensitive (femtofarad level)analog channel 500. Signal mixer 504 is essentially a synchronousrectifier as the frequency of the signal at its inputs is the same, andas a result, signal mixer output 514 can be a rectified Gaussian sinewave.

Offset compensation 506 can then be applied to signal mixer output 514,which can remove the effect of the static Csig, leaving only the effectof ΔCsig appearing as result 524. Offset compensation 506 can beimplemented using offset mixer 530. Offset compensation output 522 canbe generated by rectifying Fstim 516 using rectifier 532, and mixing therectifier output 536 with an analog voltage from digital-to-analogconverter (DAC) 520 in offset mixer 530. DAC 520 can generate the analogvoltage based on a digital value selected to increase the dynamic rangeof analog channel 500. Offset compensation output 522, which can beproportional to the analog voltage from the DAC 520, can then besubtracted from signal mixer output 514 using subtractor 534, producingsubtractor output 538 which can be representative of the change in theAC capacitance ΔCsig that occurs when a capacitive sensor on the rowbeing stimulated has been touched. Subtractor output 538 is thenintegrated and can then be converted to a digital value by ADC 508. Insome embodiments, integrator and ADC functions are combined and ADC 508may be an integrating ADC, such as a sigma-delta ADC, which can sum anumber of consecutive digital values and average them to generate result524.

FIG. 6 a is a graph of an exemplary sensor panel cross-section for onerow of sensors versus sensor output value, with solid line 600representing raw baseline no-event sensor values obtained after scanningthe row during boot-up of a sensor panel subsystem and capturing theoutput values from the analog channels connected to the columns of thesensor panel. (It should be understood that a single row is illustratedfor purposes of explanation only, but that similar graphs can begenerated for every row in the sensor panel.) As mentioned above, theelectrical characteristics of the sensors (pixels) in a sensor panel arelikely to vary due to processing variations, manufacturing tolerancesand assembly differences, resulting in variations in the sensor outputvalues as generated by the analog channels. Accordingly, in the exampleof FIG. 6 a, raw baseline no-event sensor values 600 vary from sensor tosensor across the row due to these variations. Because the raw baselineis non-uniform, a subsequent touch or hover event (after boot-up iscomplete) that occurs at or above the row can produce sensor outputvalues 604 that do not stand out appreciably from raw baseline 600, andtherefore can be difficult to detect.

To normalize the sensors, the difference (offset) between raw baselineno-event sensor value 600 and a selected normalized value for eachsensor or pixel can be computed after the boot-up scan. These offsets(one for each sensor) can then be subtracted from all subsequentlyobtained output values for that sensor (after boot-up is complete) tonormalize the sensor output values. In the example of FIG. 6 b, thelowest raw baseline value (from sensor 3 in the example of FIG. 6 a) hasbeen chosen as normalized value 602, and thus the offsets for eachsensor are computed based on this normalized value. However, it shouldbe understood that any normalized value can be selected. For example, ifthese offsets are subtracted from the raw baseline values, all sensoroutput values will be adjusted to approximately normalized value 602,producing a substantially flat normalized response. With the offsetscomputed for each sensor, the same subsequent touch or hover event asshown in FIG. 6 a will now produce normalized sensor output values 606that stand out more appreciably from the normalized baseline 602, makingit easier to detect.

The examples of FIGS. 6 a and 6 b presume that no finger or other objectwas touching or hovering over the sensor panel at the time of theboot-up scan. However, as illustrated in FIG. 6 c, if a finger or otherobject was touching or hovering over the sensor panel during boot-up,captured raw baseline 608 will have erroneously higher values ascompared to correct raw baseline 600. As a result, when offsets arecomputed and subtracted from erroneous raw baseline 608, an erroneousnormalized baseline 610 will be produced. When the finger or otherobject is removed, the no-touch sensor values will appear as a negativecurve (a so-called “negative finger”) 612, as shown in FIG. 6 d.

One conventional solution to this problem is to perform a“minimum-increment baseline adaptation” process which periodically scansfor the minimum sensor values and updates the raw baseline to reflectany newly obtained minimum sensor values. Accordingly, if a finger orother object was initially present during the boot-up scan and anerroneously high raw baseline was obtained, when the finger or otherobject is removed, a subsequent scan is performed and some sensor outputvalues drop, those lower output values become the new raw baselinevalues for the affected sensors. However, due to expected butunpredictable fluctuations in the sensor readings, the raw baselines areoccasionally incremented by a fixed amount so that the raw baseline doesnot become artificially low due to the fluctuations. The approach tendsto track the bottom of the analog channel noise envelope. One problemwith this approach is that a finger or other object initially presentduring the boot-up scan will corrupt the raw baseline and therefore thesensor readings until the finger or other object is removed and a newraw baseline is obtained.

FIG. 7 is a flowchart illustrating baseline normalization according tosome embodiments of the invention in which pre-stored no-touch orno-hover (no-event) sensor output values can initially be used when asensor panel subsystem is first booted up to establish an initialbaseline of sensor output values unaffected by fingers or other objectstouching or hovering over the sensor panel during boot-up. By using thepre-stored baseline until the finger or other object is removed, thebaseline is not initially corrupted, and the presence or absence of afinger or other object can be more easily detected. It should beunderstood that most of the steps of FIG. 7 can be performed by thecomputing system of FIG. 1, under control of baseline normalizationfirmware or software 140 executed in panel processor 102, and/orbaseline normalization hardware 138 in panel subsystem 106.

In FIG. 7, the raw no-event baseline values for the sensor panel can becaptured prior to final assembly, such as at the factory, for example,at step 700, and can be stored in non-volatile memory such as an EEPROMat step 702. During boot-up, rather than scanning the sensor panel forthe raw baseline, the pre-stored raw baseline, unaffected by any fingeror object that may be present, can be used at step 704. Note that thepre-stored raw baseline may have been captured at a temperature that isdifferent from the ambient temperature during boot-up, but neverthelessany error in the pre-stored raw baseline due to temperature coefficientsis expected to be small. Optionally, if ambient temperature data and atemperature coefficient is available, the factory baseline values couldbe adjusted for the proper temperature at 706. Offsets can then becomputed so that they can be applied to subsequently received outputvalues (after boot-up is complete) to generate normalized output valuesat step 708.

A scan of the sensor panel to obtain raw sensor output values can thenbe performed at step 720. A finger or other object can then be detectedat step 710 because the raw sensor output values will be higher than thepre-stored raw baseline. Alternatively, it can be determined that thereis no finger of other object present due to the appearance of relativelyflat raw output values (within a predetermined range of each other), orif the output values are below some predetermined threshold or withinsome predetermined range of the pre-stored raw baseline. If a finger orother object is in fact detected, the process waits at step 712 untilthe finger of other object has been removed, and the values appear flatand/or fall below the threshold or within the predetermined range of thepre-stored raw baseline. When it has been determined that no finger orother object is present, a new scan of the sensor panel can be performedat step 722, and a new baseline can be obtained and the pre-stored rawbaseline can be discarded at step 714. If no finger or other object isdetected at step 710, a new baseline can be obtained and the pre-storedraw baseline can be discarded at step 714.

In some embodiments of the invention, the detection of a finger or otherobject at step 712 can be used at optional step 716 to initiatediagnostics modes that run internal tests on the device, or put thedevice into a certain operational mode. For example, suppose the sensorpanel is employed in a mobile telephone, and the telephone is in a sleepmode in which the sensor panel is powered down (but not the wholetelephone). When a telephone call is received, the sensor panel powersup and an “answer phone” virtual button is displayed beneath the sensorpanel. If a finger or other object happens to be touching or hoveringover that portion of the sensor panel covering the “answer phone”virtual button when the call is received, the user would normally haveto remove the finger or other object, wait for a new raw baseline to beobtained, and then put the finger or other object back down over thesensor panel to trigger the virtual button. However, in otherembodiments of the invention, if a finger or other object is detected,the “answer phone” virtual button may be automatically triggered so thatthe user can answer the phone without having to first lift his/herfinger off and then place it back down. Other tasks can be triggered aswell. For example, if a multi-event sensor panel detects a number offingers, this may be interpreted as a sign that the user wants to typesomething, so a full keypad function may be activated.

In some embodiments of the invention when a pre-stored raw baseline isnot employed as described above, an enhancement to the “minimumincrement baseline adaptation” process described above may be performedinstead. The enhancement is based on the empirical observation that if alarge object such as a palm is placed on a sensor panel, the sensoroutput values underneath the large object will increase as expected, butjust outside the object, where there is no touching or hovering, thesensor output values will decrease. FIG. 8 a illustrates thisphenomenon. In FIG. 8 a, a large object touch or hover event 800 causesthe output values of the sensors under the large object to increase, asexpected, but output values 802 of the sensors just outside the largeobject unexpectedly drop below normalized baseline 804. If theconventional “minimum increment baseline adaptation” process is used,the raw baseline values for these outside sensors will be artificiallypushed down, and an inaccurate revised baseline will be generated.

To overcome this problem, some embodiments of the present invention canemploy a global baseline inversion detection algorithm during boot-up.In this algorithm, illustrated in FIG. 8 b, a scan of the entire sensorpanel is performed and a baseline is captured at step 806. In someembodiments, multiple (e.g. four) consecutive image scans can beperformed, and averaged raw sensor output values can be obtained tocreate a baseline with extra precision to allow for more gradualadaptation. Next, the number of sensor output values in the entire imagewith values above the current normalized baseline value are summed(above_baseline_sum) at step 808, and the number of sensor output valuesbelow the normalized baseline are summed (below_baseline_sum) at step810. A determination is made whether the magnitude of below_baseline_sumis much greater than the magnitude of above_baseline_sum (greater than apredetermined margin) for several frames at step 812. If this conditionis not satisfied, the sensor panel is re-scanned at step 816, and steps808, 810 and 812 are repeated. The baseline is recaptured at step 814only when the magnitude of below_baseline_sum is >>magnitude ofabove_baseline_sum, and this condition persists for several consecutiveframes. In one embodiment, this condition must persist for a certainnumber of consecutive frames (e.g. 16), and thereafter the baseline isrecaptured from the next several (e.g. four) frames. It should beunderstood that the steps of FIG. 8 b can be performed by the computingsystem of FIG. 1, under control of baseline normalization firmware ofsoftware 140 executed in panel processor 102, and/or baselinenormalization hardware 138 in panel subsystem 106.

When a palm or other large object is present, the captured sensor outputvalues will be high for the sensors underneath the large object and lowfor the outside sensors, and an erroneous baseline will be captured. Aslong as the palm or other large object is present, because the erroneousbaseline is being used, the values for above_baseline_sum andbelow_baseline_sum will both be small, the equation of step 812 will notbe satisfied, and no new baseline will be captured. As a result, thebaseline values for the sensors outside the large object will not bepushed down to an artificially low level. When the palm or other largeobject is removed, all output values will return to the correctbaseline. In other words, the output values for the sensors that wereunderneath the large object will drop, and the output values for thesensors outside the large object should rise. However, because that thecaptured erroneous baseline is still being used, the magnitude ofbelow_baseline_sum will be much greater than the magnitude ofabove_baseline_sum. If this condition persists for a few scans, thealgorithm concludes that the palm or other large object has definitivelybeen removed, and a new baseline can be captured at 814.

Detecting baseline inversion as described above using above_baseline_sumand below_baseline_sum may be sufficient to recover from whole fingerson the surface during original baseline capture. However, it may nottrigger a recapture from partial fingers that were along an edge, orfrom non-grounded conductors like coins and keys that produce a mix ofnegative and positive sensor output values. To handle these cases, insome embodiments the global baseline inversion detection algorithm canbe modified to trigger recapture based on the detection of isolated,slightly below baseline sensor output values even in the presence ofmany weakly positive sensor output values. As illustrated in FIG. 8 c,to prevent background noise from spuriously triggering such recaptures,the negative sensor output value for any sensor may be required to bebelow the baseline output value for that sensor by some amount N (seestep 818), where N may be chosen with sufficient margin to avoidtriggering caused by a background noise standard deviation, such as fouror five standard deviations of pixel or background noise below thebaseline output value for that sensor. (Note that the peak pixelthreshold for starting a watershed patch (i.e. starting the tracking ofa touch event) is also about 4-5 standard deviations of background noiseabove the baseline output value for that sensor). Note that step 818 maybe performed at any time.

In addition, the modified algorithm can optionally require that anyindividual positive touch sensor output values be limited to not morethan some threshold level, such as a particular level representative ofhover strength (i.e., positive sensor output values caused by afingertip hovering over the surface within the near-field detectioncapabilities of the touch sensor, which are generally weaker than thetouch threshold), and additionally that the sum of above baseline sensoroutput values does not exceed M times the sum of below baseline sensoroutput values (see step 820), where M may be eight, for example. Theseconditions effectively prevent baseline recapture from slightly negativesensor output values during a finger touch.

Water and other conductive solutions can cause negative imageperturbations with no positive component. These wholly negative sensoroutput value regions can trigger a recapture due to the global baselineinversion detection algorithm described above. As the solution driesoff, wholly positive, false sensor output value patches that areindistinguishable from objects hovering within the near-field detectioncapabilities of the touch sensor) may be left over. However, these falsepatches may have no negative sensor output values to trigger arecapture. Therefore, in some embodiments of this invention, the globalbaseline inversion detection algorithm of either FIG. 8 b or 8 c can befurther modified to include a timeout rule, which provides that anyunexpectedly sustained (e.g. for 10 seconds) positive patches (i.e.patches that appear to be caused by a finger hovering within thenear-field detection capabilities of a touch sensor) trigger arecapture. Users are unlikely to intentionally sustain a finger hoverwithin the near-field detection capabilities of a touch sensor in thesame spot for more than a few seconds, so this rule should not have anoticeable impact on user experience. This modification can also act asa fail-safe if periodic baseline adjustments (described below) fail tokeep up with a particularly severe temperature ramp. Note that thedetection of a hover event within the near-field detection capabilitiesof a touch sensor should not trigger a re-capture when optical proximityis also detected (e.g. when a separate proximity sensor detects thepresence of a user's head), otherwise the optical baseline could getreset within 10 seconds as well. Thus, the sustained hover rule can beapplied only when optical proximity is not above its detectionthreshold. This can lessen the likelihood that hovering ears and cheekswill be erased by a timeout recapture.

FIG. 8 d illustrates an exemplary periodic baseline adjustment algorithmaccording to some embodiments of this invention. The periodic baselineadjustment algorithm can increment or decrement individual baselinesensor output values by one count or unit, or some small value. Thisperiodic baseline adjustment algorithm can be implemented independent ofany other baseline algorithm described above. One purpose of thisadjustment mode is to provide periodic fine-tuning of the baseline totrack temperature drift.

To perform periodic baseline adjustment, at step 822 a scan of thesensor panel is performed after a dynamic adjustment time interval haspassed. This time interval is generally much longer than the frame rate(the time it takes to scan the entire sensor panel one time). Previouslycomputed offset values are subtracted from the sensor output values tonormalize them at step 824, and all normalized sensor output values thatare positive and negative are identified at step 826. For any normalizedsensor values that are positive, the offset values are incremented by Pat step 828, where P may be one count, or a small value, or a percentageof the positive value. For any normalized sensor values that arenegative, the offset values are decremented by Q at step 830, where Qmay be one count, or a small value, or a percentage of the negativevalue. The adjustment interval may be modified at step 832, and then thealgorithm waits the duration of the adjustment period before scanningthe panel again at step 822.

The adjustment interval can vary. Dynamic adjustment intervals allowquick recovery for slight baseline inversions (e.g. heating on columnsw/ negative temperature coefficient) without adapting out far-field ortouch objects very quickly. Adjustment intervals can depend on anaverage of background (non-patch) sensor output values (similar tofar-field measurement but linear), computed on a periodic basis than isgenerally much faster then the adjustment interval. This backgroundaverage represents the average sensor output values in non-touch ornon-hover areas (i.e. outside of “patch” areas apparently caused bytouch or hover events). One exemplary algorithm for modifying theadjustment interval in step 832 is as follows, although it should beclearly understood that the values listed therein are for purposes ofillustration only, and other values can be used in accordance with theinvention:

  If background average negative  adjust interval = 250ms (handles fastwarm-up); else if background average took > 1 second to drift positive adjust interval = 1 second (to handle fast cool-down); else ifbackground average neutral or suddenly positive  if Small Hover detectedin previous frame   adjust interval = 8 seconds,  else if Finger Touchdetected in previous frame   adjust interval = 256 seconds,  else  adjust interval = 4 seconds (default).

If the background average is negative, this can be an indication thatthe sensor panel is warming up quickly, and thus the adjustment intervalcan be set to a first interval, such as 250 ms. If the backgroundaverage took more than a second interval (larger than the firstinterval), such as one second, to drift positive, this can be anindication of a fast sensor panel cool-down, and thus the adjustmentinterval can be set to that second interval. If the background averagetook less than the second interval to drift positive, this can be anindication of a large object. If the background average is neutral orsuddenly positive (e.g. drifted positive in less than the secondinterval), then if a small hover was previously detected, indicating apossible upcoming touch event, the adjustment interval can be set to afourth interval which is longer than the second interval. If a fingertouch was previously detected, indicating the likelihood of multipletouch events in the near future, the adjustment interval can be set to afifth interval which is longer than the fourth interval. If neitherevent was dictated, the adjustment interval can be set to a thirdinterval, which is between the second and fourth intervals in duration.In some embodiments, to be considered negative, the background averagecan be about one standard deviation of background noise below neutral.To be considered positive it can be greater than about 2 standarddeviations of background noise above neutral.

FIG. 9 a illustrates an exemplary mobile telephone 936 that may includemulti-touch panel 924 and/or multi-hover panel 934, and panel subsystemand panel processor 930 configured for implementing baselinenormalization or global baseline inversion detection as described aboveaccording to embodiments of the invention. FIG. 9 b illustrates anexemplary digital audio/video player 938 that may include multi-touchpanel 924 and/or multi-hover panel 934 and panel subsystem and panelprocessor 930 configured for implementing baseline normalization orglobal baseline inversion detection as described above according toembodiments of the invention. The mobile telephone and digitalaudio/video player of FIGS. 9 a and 9 b can advantageously benefit frombaseline normalization or global baseline inversion detection becausewithout it, fingers or other objects present on or over the sensor panelduring boot-up can cause erroneous baselines to be computed, which canfurther cause errors in the detection of touch or hover events on thesensor panel and inconsistent or erroneous triggering of virtualbuttons.

Although the present invention has been fully described in connectionwith embodiments thereof with reference to the accompanying drawings, itis to be noted that various changes and modifications will becomeapparent to those skilled in the art. Such changes and modifications areto be understood as being included within the scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for determining an adjustment intervalfor a touch sensor panel, comprising: performing a scan of the touchsensor panel to obtain sensor output values; identifying non-touchsensor output values indicative of an absence of a touch or hover event;computing an average of the non-touch sensor output values; and choosinga time interval for adjusting subsequent sensor output values as afunction of the computed average.
 2. The method of claim 1, wherein theadjusting of subsequent sensor output values includes baseline driftcompensation.
 3. The method of claim 1, further comprising choosing theadjustment interval to be a first time interval if the computed averageis negative.
 4. The method of claim 3, further comprising: if thecomputed average took longer than a second time interval to change fromnegative to positive, setting the adjustment interval to the second timeinterval, the second time interval being longer than the first timeinterval; and if the background average is neutral or took less than thesecond time interval to change from negative to positive, then if ahover event was detected in a previous touch sensor panel scan, settingthe adjustment interval to a fourth time interval longer than the secondtime interval, if a touch event was detected in the previous touchsensor panel scan, setting the adjustment interval to a fifth timeinterval longer than the fourth time interval, and if neither a hovernor touch event was detected in the previous touch sensor panel scan,setting the adjustment interval to a third time interval, the third timeinterval between the second and fourth time intervals in duration. 5.The method of claim 4, further comprising determining that the computedaverage is positive when the computed average is about two standarddeviations above neutral.
 6. The method of claim 3, further comprisingdetermining that the computed average is negative when the computedaverage is about one standard deviation below neutral.
 7. The method ofclaim 1, further comprising: setting the adjustment interval to a firsttime interval if the computed average indicates that the touch sensorpanel is warming up.
 8. The method of claim 7, further comprising:setting the adjustment interval to a second time interval longer thanthe first time interval if the computed average indicates that the touchsensor panel is cooling down; and if the computed average is neutral ortook less than the second time interval to change from negative topositive, then setting the adjustment interval to a fourth time intervallonger than the second time interval if a previous touch sensor panelscan indicates an occurrence of a hover event, setting the adjustmentinterval to a fifth time interval longer than the fourth time intervalif the previous touch sensor panel scan indicates an occurrence of atouch event, and if neither a hover nor a touch event was detected inthe previous touch sensor panel scan, setting the adjustment interval toa third time interval, the third time interval between the second andfourth time intervals in duration.
 9. An apparatus for determining anadjustment interval for a touch sensor panel, comprising: a processorcapable of performing a scan of the touch sensor panel to obtain sensoroutput values; identifying non-touch sensor output values indicative ofan absence of a touch or hover event; computing an average of thenon-touch sensor output values; and choosing a time interval foradjusting subsequent sensor output values as a function of the computedaverage.
 10. The apparatus of claim 9, wherein the adjusting ofsubsequent sensor output values includes baseline drift compensation.11. The apparatus of claim 9, wherein the processor is further capableof choosing the adjustment interval to be a first time interval if thecomputed average is negative.
 12. The apparatus of claim 11, theprocessor further capable of: if the computed average took longer than asecond time interval to change from negative to positive, setting theadjustment interval to the second time interval, the second timeinterval being longer than the first time interval; and if thebackground average is neutral or took less than the second time intervalto change from negative to positive, then if a hover event was detectedin a previous touch sensor panel scan, setting the adjustment intervalto a fourth time interval longer than the second time interval, if atouch event was detected in the previous touch sensor panel scan,setting the adjustment interval to a fifth time interval longer than thefourth time interval, and if neither a hover nor touch event wasdetected in the previous touch sensor panel scan, setting the adjustmentinterval to a third time interval, the third time interval between thesecond and fourth time intervals in duration.
 13. The apparatus of claim12, the processor further capable of determining that the computedaverage is positive when the computed average is about two standarddeviations above neutral.
 14. The apparatus of claim 11, the processorfurther capable of determining that the computed average is negativewhen the computed average is about one standard deviation below neutral.15. The apparatus of claim 9, the processor further capable of: settingthe adjustment interval to a first time interval if the computed averageindicates that the touch sensor panel is warming up.
 16. The apparatusof claim 15, the processor further capable of: setting the adjustmentinterval to a second time interval longer than the first time intervalif the computed average indicates that the touch sensor panel is coolingdown; and if the computed average is neutral or took less than thesecond time interval to change from negative to positive, then settingthe adjustment interval to a fourth time interval longer than the secondtime interval if a previous touch sensor panel scan indicates anoccurrence of a hover event, setting the adjustment interval to a fifthtime interval longer than the fourth time interval if the previous touchsensor panel scan indicates an occurrence of a touch event, and ifneither a hover nor a touch event was detected in the previous touchsensor panel scan, setting the adjustment interval to a third timeinterval, the third time interval between the second and fourth timeintervals in duration.
 17. The apparatus of claim 9, the apparatuscoupled to the touch sensor panel.
 18. The apparatus of claim 9, theapparatus incorporated into a mobile telephone.
 19. The apparatus ofclaim 9, the apparatus incorporated into a digital audio player.
 20. Anon-transitory computer-readable storage medium comprising program codefor determining an adjustment interval for a touch sensor panel, theprogram code for causing performance of a method comprising: performinga scan of the touch sensor panel to obtain sensor output values;identifying non-touch sensor output values indicative of an absence of atouch or hover event; computing an average of the non-touch sensoroutput values; and choosing a time interval for adjusting subsequentsensor output values as a function of the computed average.
 21. Thenon-transitory computer-readable storage medium of claim 20, wherein theadjusting of subsequent sensor output values includes baseline driftcompensation.
 22. The non-transitory computer-readable storage medium ofclaim 20, the method further comprising choosing the adjustment intervalto be a first time interval if the computed average is negative.
 23. Thenon-transitory computer-readable storage medium of claim 22, the methodfurther comprising: if the computed average took longer than a secondtime interval to change from negative to positive, setting theadjustment interval to the second time interval, the second timeinterval being longer than the first time interval; and if thebackground average is neutral or took less than the second time intervalto change from negative to positive, then if a hover event was detectedin a previous touch sensor panel scan, setting the adjustment intervalto a fourth time interval longer than the second time interval, if atouch event was detected in the previous touch sensor panel scan,setting the adjustment interval to a fifth time interval longer than thefourth time interval, and if neither a hover nor touch event wasdetected in the previous touch sensor panel scan, setting the adjustmentinterval to a third time interval, the third time interval between thesecond and fourth time intervals in duration.
 24. The non-transitorycomputer-readable storage medium of claim 23, the method furthercomprising determining that the computed average is positive when thecomputed average is about two standard deviations above neutral.
 25. Thenon-transitory computer-readable storage medium of claim 22, the methodfurther comprising determining that the computed average is negativewhen the computed average is about one standard deviation below neutral.